Recently, with an increase in size of structures, a reduction in weight of structural members has become important. In order to realize this, an effort has been made to increase the tensile strength of a steel plate used in the structures. Since, however, ships, offshore structures, bridges, and the like repeatedly undergo loading during use, consideration should be given to the prevention of fatigue failure. Welds are sites where a fatigue fracture is most likely to occur, which has led to a demand for an improvement in fatigue strength at the weld.
Up to now, the factors governing the fatigue strength at the weld and an improvement in the fatigue strength have been studied, and an improvement in fatigue strength at the weld has been primarily attempted by mechanical factors, such as a reduction in stress concentration through an improvement in the shape of the toe of the weld such as shaping of the toe of weld by grinding using a grinder or heat-remelting of the final layer of the weld bead, or shot peening treatment or other treatments for creating compressive stress at the toe of weld (Japanese Unexamined Patent Publication (Kokai) Nos. 59-110490 and 1-301823 and the like). Further, it is well known that the effect of reducing the residual stress can be attained by post-weld heat treatment.
On the other hand, a proposal has been made wherein the fatigue strength at a weld is improved by taking advantage of chemical compositions of steel products without use of the above special execution and post-weld heat treatment.
In Japanese Unexamined Patent Publication (Kokai) No. 62-10239, in order to prevent a deterioration in fatigue properties at a spot weld even in the case of high C and high Mn levels by increasing the Si content and specifying the amounts of C and P added, a high-strength thin steel sheet having excellent fatigue properties in spot welding, comprising C: not more than 0.3%, Si: 0.7 to 1.1%, Mn: not more than 2.0%, P: not more than 0.16%, and sol. Al: 0.02 to 0.1%, is disclosed.
In Japanese Unexamined Patent Publication (Kokai) No. 3-264645, in order to attain good stretch-flange ability, fatigue properties, and resistance weldability by advantageously forming clean polygonal ferrite by Si, strengthening and improving the hardenability of a steel by B, a high-strength thin steel sheet having excellent stretch-flange ability and other properties, comprising C: 0.01 to 0.2%, Mn: 0.6 to 2.5%, Si: 0.02 to 1.5%, B: 0.0005 to 0.1%, and the like, is disclosed.
In Japanese Examined Patent Publication (Kokoku) No. 3-56301, in order to advantageously improve the fatigue strength of a joint at its spot weld by optimizing the chemical compositions in the steel and the proportion of unrecrystallized structure in the steel sheet by adding B or the like, a very low carbon steel plate having a good spot weldability, comprising C: not more than 0.006%, Mn: not more than 0.5%, Al: not more than 0.05%, and 0.001 to 0.100% in total of at least one member selected from Ti and/or Nb in a solid solution form exclusive of a nitride and a sulfide, is disclosed.
Among the above techniques, the techniques disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 59-110490 and 1-301823 requires special execution after welding and cannot improve the fatigue strength of the as-welded steel. The technique where heat treatment is carried out after welding requires additional steps and unfavorably complicates welding procedure. Further, the effect attained by the technique is limited.
The thin steel sheets disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 62-10239 and 3-264645 are those of which the applications are mainly limited to base materials of wheels and disks for automobiles, and these steel sheets are quite different from steel plates used in shipbuilding and offshore structures, contemplated in the present invention, in applications, plate thickness, and use. Therefore, the findings associated with these steel sheets, as such, cannot be applied to the steel plates. Also regarding steel chemical compositions, the thin steel sheet disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-10239 specifies particularly the relationship between the C and P contents to C: less than 0.22%, P: not more than 0.16%, and C: 0.22 to 0.3% with C +0.6P.ltoreq.0.31 from the viewpoint of improving the fatigue strength at its spot weld, and this publication is utterly silent on solid-solution strengthening of a ferritic structure at a weld formed by arc welding.
Specifically, spot welding is a kind of resistance welding and used mainly in welding of thin steel sheets having a sheet thickness in the range of from about 0.5 to 3.5 mm after forming, for example, welding of thin steel sheets for members of automobiles. In the spot welding, portions to be welded are clamped between electrodes, and a large current is passed through the assembly for a short time.
Therefore, the spot welding is different from arc welding used in welding of high-tensile steel plates, having a thickness of not less than 6 mm, as materials for shipbuilding, offshore structures, bridges, and the like in welding process, such as shape of electrodes, use or not of welding materials, and welding conditions, as well as in the shape of the weld, the weld residual stress, and the like, resulting in a difference in factors governing the fatigue strength between both the welding methods. Thus, even though the fatigue strength could be improved in spot welding, the findings for spot welding, as such, cannot be applied to arc welding.
On the other hand, for the thin steel sheet disclosed in Japanese Unexamined Patent Publication (Kokai) No. 3-264645, B is added to improve the strength and hardenability of the steel, thereby providing a desired structure. This publication is silent on the relationship between the addition of B and the weldability. Further, no mention is made of an improvement in fatigue strength of welds besides base materials.
Japanese Examined Patent Publication (Kokoku) No. 3-56301 describes a spot weld of a very low carbon steel sheet and aims to regulate the hardness distribution at a spot weld. In this steel sheet, B is added to refine the structure and prevent grain growth. The upper limit of the amount of B added is set from the viewpoint of preventing a deterioration in material, and no study is made of the weldability.
An object of the present invention is to improve the fatigue strength of a weld of structural members, particularly a weld formed by arc welding.
Another object of the present invention is to improve the fatigue strength of structural members at their welds, particularly a weld heat affected zone (hereinafter referred to as "HAZ") of structural members by regulating the HAZ micro-structure of the as-welded structural members.
A further object of the present invention is to provide a high-tensile steel plate having weldability good enough to stop weld cracking upon welding.
A further object of the present invention is to provide a process for producing a high-tensile steel plate which can attain the above object.